Vacuum brake boosters have been known for a long time, for example from DE 44 05 092 C1, and corresponding U.S. Pat. No. 5,493,946 which is incorporated by reference herein, and are used in a vast number of cases to boost the actuating forces of hydraulic vehicle brake systems and thereby maintain them at a level which is comfortable for the driver of a vehicle.
Also known from DE 44 05 092 C1 (U.S. Pat. No. 5,493,946) is a so-called brake assistant. This term is usually understood to mean a system which can make increased braking power available to a driver with substantially the same actuating force in an emergency braking situation. Systems of this kind were developed because tests revealed that, although when emergency braking most vehicle users depress the brake pedal quickly, they do not do so forcefully enough to achieve the maximum possible braking power. The stopping distance of the vehicle is therefore longer than necessary. Systems of this kind which are already in production use a brake booster adapted to be electromagnetically actuated together with a device adapted to determine the actuating speed of the brake pedal. If this device detects an actuating speed lying above a predetermined threshold value, it is assumed that an emergency braking situation exists and the brake booster is fully driven by means of the electromagnetic actuating device, i.e. it provides its maximum boost power.
DE 44 05 076 and corresponding U.S. Pat. No. 5,483,866 which is incorporated by reference herein, describes a pneumatic brake booster with electromagnetically initiated brake assistant function. If an actuating member 44 is released while a valve seat 32 formed on a valve body 40 is, by an electromagnet 40, retained in a position in which it is remote from a sealing member 60, a further valve seat 72 abuts the sealing member 60 which deactivates the brake assistant.
However, brake boosters with an electromagnetic actuation facility are too expensive for motor vehicles of the lower and middle price category. Solutions which achieve a brake assistant function at less expense are therefore required.
DE 198 24 501 A1 describes a vacuum brake booster with pneumatically controlled brake assistant function. When a brake actuating rod 28 is actuated rapidly, a pressurized chamber 52 is connected to the ambient air. As a result, a closing piston 41 is moved opposite to the direction of actuation of the brake actuating rod 28, and a third sealing seat 37 blocks the connection between the vacuum chamber 29 and the working chamber 35 even if a first sealing seat 27 is lifted off the valve body 25 due to the movement of a control housing 11. This arrangement prevents that the movement of the control housing 11 results in the boosting effect stopping already before the working chamber is completely filled with atmospheric pressure.
The object of the invention is to provide a vacuum brake booster of the above-mentioned type with a brake assistant function without having to resort to a control valve adapted to be electromagnetically actuated. At the same time, on the one hand unintentional activation of the brake assistant function is to be prevented, as far as this is possible, and on the other hand, once the brake assistant function has been initiated, it should be possible to deactivate it in a convenient and reliable manner.
According to the invention, the first valve seat is formed at a displaceable valve member which is coupled to the input member in the actuating direction of the brake booster. The displaceable valve member is always subjected to the pressure prevailing in the working chamber at its rear side, which is turned towards the input member. If, however, the displaceable valve member is displaced relative to the control valve housing in the actuating direction by more than a prefixed distance, the displaceable valve member is subjected over at least a part of its front side, which lies opposite the rear side, to the pressure prevailing in the vacuum chamber, and the pressure difference then acting on the displaceable valve member holds the valve member in the position reached until pressure equalization between the front side and the rear side of the displaceable valve member takes place. According to the invention, therefore, the pressure difference prevailing at the mobile wall at the time is used to exert a force acting on the displaceable valve member in the actuating direction in order to hold the first valve seat formed at the displaceable valve member completely open in certain situations in which the actuating force exerted by the driver is not sufficient for this, so that the brake booster builds up the greatest possible pressure difference between its vacuum chamber and its working chamber, i.e. provides its maximum boost power.
After exceeding the above-mentioned, prefixed relative displacement, the displaceable valve member of the vacuum brake booster according to the invention is therefore virtually xe2x80x9csuckedxe2x80x9d into a position in which the first valve seat is completely open. However, this necessary relative displacement is only achieved if the actuating speed of the input member exceeds a defined value. In the vacuum brake booster according to the invention the brake assistant function is therefore activated solely by the skilful utilization of pressure differences present within the brake booster. No magnet is required to initiate the brake assistant function.
In the vacuum brake booster according to the invention the brake assistant function is disengaged by means of a reduction in the pressure difference acting on the displaceable valve member. The reduction in this pressure difference is initiated by a return movement of the input member which exceeds a certain extent, and a component which is coupled to the input member in the direction opposite to the actuating direction of the brake booster acts on the first valve sealing member and lifts it off the second valve seat after a prefixed displacement of the input member in the direction opposite to the actuating direction. The vacuum chamber is thereby connected to the working chamber and the pressure difference acting on the displaceable valve member is reduced, which results in a reliable and convenient deactivation of the brake assistant function. The prefixed displacement ensures that the brake assistant function is not disconnected unintentionally too soon.
In a preferred embodiment of the brake booster according to the invention the component which is coupled to the input member in the direction opposite to the actuating direction of the brake booster is a fifth valve seat which is connected to a transmission piston and preferably disposed concentrically with the first valve seat and the second valve seat.
In preferred embodiments of the vacuum brake booster according to the invention the displaceable valve member is resiliently preloaded opposite to the actuating direction of the brake booster. This resilient preload advantageously ensures that the displaceable valve member is coupled to the input member in the actuating direction of the brake booster and on the other hand enables the displaceable valve member to be uncoupled from the input member when the brake assistant function is activated. In embodiments of this kind the force acting on the displaceable valve member on account of the pressure difference must be greater than the opposing spring force acting on the valve member in order to initiate the brake assistant function. This requirement can easily be complied with by appropriately dimensioning the surfaces of the valve member which are subjected to the pressure difference.
A third valve seat is preferably formed on the displaceable valve member in the vacuum brake booster according to the invention. Said third seat co-operates with a second valve sealing member which in turn co-operates with a fourth valve seat establishing a connection between the working chamber and the vacuum chamber in the open state. After the valve member has exceeded the prefixed displacement relative to the control valve housing, the third valve seat is closed and the fourth valve seat open, so that the pressure prevailing in the vacuum chamber can now act on the displaceable valve member.
In particularly preferred embodiments is the third valve seat formed on the front side of the displaceable valve member. An annular cavity is defined between the third valve seat and the fourth valve seat, both of which are preferably annular, the front end boundary of which cavity is constituted on one side by the displaceable valve member. The fourth valve seat is in particular disposed concentrically with and radially outside of the third valve seat. The annular cavity is connected to the vacuum chamber when the third valve seat is closed and the fourth valve seat open, while it is connected to the working chamber when the third valve seat is open and the fourth valve seat closed. The surface of the displaceable valve member which is located radially between the third valve seat and the fourth valve seat can thus be subjected either to the pressure in the vacuum chamber or the pressure in the working chamber.
In preferred embodiments of the brake booster according to the invention, the second valve sealing member is resiliently preloaded opposite to the actuating direction of the brake booster and can be axially displaced against this spring preload. The extent of the axial displaceability of the second valve sealing member in this case represents the switching threshold which must be overridden in order to initiate the brake assistant function. In embodiments of this kind the force resulting from the pressure difference at the displaceable valve member must be greater than the sum of the spring forces which act in the opposite direction and which preload the second valve sealing member or the displaceable valve member in order to initiate the brake assistant function.
The displaceable valve member is preferably substantially sleeve-shaped in order to achieve a space-saving construction. The first valve seat is in this case formed at the end of the valve member which is turned towards the input member, while the third valve seat is located at the opposite end of the valve member. A sleeve-shaped, displaceable valve member of this kind can be integrated into conventional control valve constructions without noticeably affecting the diameter or overall length thereof.
The input member as well is preferably resiliently preloaded opposite to the actuating direction in all embodiments of the vacuum brake booster according to the invention. When the brake is released this resilient preload returns the input member to the initial position. This resilient preloading of the input member is also used to advantage in constructional terms during its return movement to the initial position to move back the above-mentioned component which is to act on the first valve sealing member to disconnect the brake assistant function relative to the displaceable valve member.